The present invention involves a method and apparatus for distributing brake pressure to the axles of a motor vehicle with an anti-lock brake system (ABS) pressure-medium brake.
In conventional brake systems for commercial vehicles, a brake which adapts automatically to the load (also known as an ALB) is generally provided for the purpose of optimum utilization of the brake system in terms of braking dynamics, and in particular also for the purpose of taking into account large differences in axle load between an empty vehicle and a laden one.
It is the object of such systems, in accordance with the lower axle load in the case of partial loading, to influence the braking forces by load-dependent reduction of the brake pressure input from the brake pedal, principally at the rear axle but, under certain pre-conditions, also at the front axle. Thus, in particular, the risk of skidding due to overbraked rear axles is lessened or eliminated. In addition, the intention is thereby to achieve optimum utilization of the brake system irrespective of the loading condition.
In air-sprung vehicles, the bellows pressure is, for example, used as a measure for the input variable indicating the axle load, while in steel-sprung vehicles the axle deflection, which is transmitted via a linkage, or the electrical signal of a pressure- or tension-sensitive electronic load transmitter connected in series with a spring bearing is generally used as shown in German Offenlegungsschrift 3,711,175.
The mechanical influencing of axle-specific brake-pressure regulating valves is susceptible to damage under tough operating conditions. An additional disadvantage of known systems is the fact that corresponding structural elements in diverse and often also structurally non-uniform variants are required even within one production series of vehicles. In addition, load detection based on a transmission linkage is imprecise, e.g. in the case of joint brake-pressure regulation of the axle groups of vehicles with leading or trailing axles or in the case of hard suspension. Such known detection also involves considerable costs.
Previously proposed systems of the above-described type or braking methods performed on them are without exception based on regulating interventions into a brake application which is in each case under way, in the sense of a current braking-force distribution recognized as inexpedient or incorrect. Here, the aim of the braking method is always the elimination of setpoint deviations which have already occurred. Such a corresponding method is disclosed in German Offenlegungsschrift 3,829,951.
Furthermore, German Offenlegungsschrift 3,323,402 describes a brake system for motor vehicles in which the brake-pedal pressure, power-assisted via pressure lines in which pressure modulators are inserted, can be transmitted to the wheel-brake cylinders. This system comprises, among other things, sensors for ascertaining the rotational behavior of the wheels and the speed of the vehicle and electronic circuits for the processing and logical combination of the measured values and for the production of actuating signals for the modulators. By way of the brake pressure modulators, both control of the brake pressure distribution as a function of the brake slip of the front wheels and brake-slip regulation which prevents locking up of the front wheels is performed.
German Offenlegungsschrift 3,226,074 describes a braking-force regulating system for motor vehicles in which a setpoint value for the deceleration of the motor vehicle is specified, the actual value is determined and the brake pressure is regulated accordingly from the deviation of the setpoint and the actual value. Regulation as a function of the slip between the roadway and the wheel is also provided.
It is an object of the present invention to provide an improved method of brake-pressure distribution to the axles of a motor vehicle with an ABS pressure-medium brake, which method leads to corresponding rotational wheel speeds and to this extent to a uniform approach to the lock-up range. The method thus provides an improvement in the range of frequent braking, i.e. already in the adaptive-braking range well below the normal range of action of an anti-lock brake system (ABS).
The foregoing objects have been achieved in accordance with the present-invention such that when a limiting value is exceeded, for intervention in individual brake applications a first regulation is, in a first step, performed dynamically in the sense of a continuously cyclic acquisition and processing of current actual wheel-speed differences for the purpose of immediate influencing of the brake-pressure distribution while the respective braking operation is still underway and in a second step, a second regulation in the manner of an adaptively predictive precontrol of the brake-pressure distribution .phi. for any retardation levels is made the basis of this first regulation. Mutually associated steady-state values of the referred inter-axle speed difference and of the retardation in the case of the old brake-pressure distribution are sought on the basis of predetermined limiting values for the time rate of change of these variables.
As soon as steady-state values are present, then, after at least one initial brake application, parameters of a brake-pressure distribution function are determined according to the present invention, in which function the setpoint deceleration demanded by the driver via the brake pedal is included. This brake-pressure distribution is taken as the basis for the subsequent braking operations. As soon as, after the determination of the parameter in the course of a subsequent brake application, a second, smaller limiting value is exceeded by a steady-state value of the referred speed difference, the old value of the brake pressure distribution function is corrected to a new value by way of the first regulation in the current cycle.
The method envisages the achievement of identical wheel speeds by adaptive regulation on two levels of action, one being steady-state and the other being dynamic. Accordingly, on one hand, an immediate dynamic intervention occurs, given sufficiently large wheel-speed differences between the axles. On the other hand, an adaptive predetermination of "correct" braking-force distributions is made the basis for each current regulating intervention. For this purpose, as a function of the retardation level selected at the brake pedal, the braking-force distribution appropriate for this in each case is determined as an estimated value, stored even before large differences in rotational speed occur, and adapted, i.e., corrected, if required in the course of subsequent dynamic braking demands.
The dynamic intervention can, for example, be advantageously carried out in practice according to the known method in accordance with the German Offenlegungsschrift 3,829,951.
Generally speaking, the method according to the present invention allows the utilization of tried and tested signal paths and transmitters of a presently commercially available ABS in order to achieve a load-dependent braking function acting well below the wheel lock limit. The method according to the invention allows higher than customary brake pressures to be fed in at the front axle independently of the control of brake pressure at the rear axle. By corresponding expansion of an appropriate microprocessor program of an electronic ABS controller, it is possible to dispense completely with the mechanical (or, in electro-pneumatic brake systems, the additional electronic) wheel or axle-load sensors.
The method according to the present invention allows an economical optimum utilization of the overall braking-power capacity of a brake system for commercial vehicles, while additionally minimizing the number of signal transmitters required and of their requisite connecting paths to an electronic control device. In this respect, the method also brings about a marked increase in reliability of a correspondingly acting brake system.